This invention relates generally to large irrigation control systems of the type used in the irrigation of agricultural land or golf courses. More specifically, the invention relates to techniques for efficient control of irrigation water by taking into account the capacity and limitations of water pumps and water distribution components through which the water flows.
In the cross-referenced applications, a system is described for the control of irrigation using evapotranspiration data derived from a weather observation station. For purposes of irrigation control, the weather data can be reduced to a single quantity referred to as ET, for evapotranspiration, measured in inches (or millimeters) of water over a 24-hour period. For a particular crop, such as a grass, the value of ET represents the amount of water that has to be spread over the crop each day to replace the moisture lost by the natural and ongoing processes of evaporation and transpiration. The cross-referenced applications provide a convenient technique for applying ET data to vary watering times qualitatively in accordance with changing weather conditions, but without necessarily using an absolute ET value for each watering station. Another feature disclosed in the cross-referenced applications was the ability to store adjustments to the ET level for different geographic zones under irrigation. Yet another important feature allows an operator of the system to define a minimum soak time and a maximum cycle time for any watering station. When this feature is used, watering time is not permitted to exceed the specified cycle time, and is not permitted to begin again until the specified soak time has elapsed, regardless of the the total watering time required as a result of the ET level or specified by the operator.
Operation under ET control relieves the operator of the burden of having to modify the control system in accordance with daily or seasonal changes in the weather. Cycle-and-soak control avoids overwatering in specific areas that are incapable of holding the water dictated by the weather and crop conditions. For example, steeply sloping ground or some types of soils may not absorb water fast enough to avoid run-off.
In the second of the cross-referenced applications, a further improvement is described, wherein the capacity of the irrigation system pump station and the capacity of each predefined watering zone are taken into account in deciding whether to grant a watering request. In large irrigation systems, water is typically pumped to the various irrigation stations by a number of water pumps that can be brought into service as needed. When a complete daily irrigation cycle is completed without regard to the pump capacities, there will often be many large peaks in pump load, as well as times when the pump load falls off to practically nothing. Pump maintenance problems are aggravated by these cyclic load conditions. It would be preferable to maintain a practically constant load on the pumps throughout a watering cycle, and the system described in the second of the cross-referenced applications largely achieves this goal.
Another difficulty that most irrigation systems totally ignore is that the amount of water that can be "thrown" at any particular watering station is limited by the network of water pipes through which the water is delivered. Although this fact is appreciated by most system operators, taking account of it in scheduling watering operations is a most difficult task. In many irrigation systems, water is delivered from the pumps into a ring-shaped header or manifold, and from this into radiating arms connecting to sprinkler heads. Calculating water flows through such a complex configuration is difficult to say the least, and may be further aggravated in an older system in which the details of the network configuration may not be accurately known.
In the second of the cross-reference applications, some account is taken of limitations in the water delivery components, in that "flow zones" are defined to have limited total capacities. Each flow zone is a collection of output valves, each with a nominal flow capacity, and the total flow zone capacity is used as a basis for limiting the granting of additional watering requests within the same zone. Unfortunately, this does not take into account the limitations imposed by the water pipes that deliver water from the pump station to the flow zones. The present invention addresses this problem.